Carbon nanofibers easily aggregate, and cannot be easily defibrated and uniformly dispersed in a matrix such as an elastomer. As a countermeasure, an innovative carbon fiber composite material producing method has been proposed that takes advantage of the elasticity and viscosity of the elastomer, and the chemical interaction of the elastomer with the carbon nanofibers to defibrate and uniformly disperse the aggregated carbon nanofibers in the elastomer under a strong shear force applied to the elastomer (see, for example, JP-A-2005-97525).
It has been found from the examination of composite materials of multi-walled carbon nanotubes and natural rubber in an immersion solvent that single fibers of defibrated and uniformly dispersed multi-walled carbon nanotubes form a continuous spatial structure (cellulation) at a filling rate of 16 mass % or more (see, for example, Swelling and Interface Analysis of Multi-walled Carbon Nanotube/Natural Rubber Composite, Carbon TANSO 2010, No. 244, 147-152). It has been revealed that such a continuous spatial structure occurs in the interface phase between the multi-walled carbon nanotube and the rubber binding to the multi-walled carbon nanotube surface, and has a high elastic modulus and a high heat resistance. However, using large quantities of multi-walled carbon nanotubes works against processibility, and tends to increase cost.
It has been proposed to mix appropriate amounts of carbon nanofiber and carbon black with an elastomer to produce a stable carbon fiber composite material that undergoes only small thermal expansion over a wide temperature range (see, for example, JP-A-2007-39649). The amount of expensive carbon nanofiber in such a carbon fiber composite material can be reduced because the carbon nanofiber and the carbon black work together to form a continuous spatial structure.